A softphone (software telephone) is an application program that enables voice over Internet Protocol (VoIP) telephone and video calls from computing devices. Existing softphones are generally not as universal or ubiquitous as traditional phone service (e.g., landline and/or mobile) since they require separate accounts and require both user endpoints to have compatible softphones. However, softphones provide various benefits over traditional phone service. For example, softphones use existing Internet bandwidth and do not require any dedicated connection for voice communication. Since a softphone does not require a dedicated channel, it uses bandwidth efficiently. As a result, softphones generally provide users with a low cost means for voice calling, especially international calling. Accordingly, users will often employ both traditional phone service and softphones to meet their communication needs.
Without the seamless integration of mobile phones and softphones, when switching between devices after a call has already been initiated, users have to end the call on their initial device and then restart the call on their second device. For example, when a user switches a call established with another person from her mobile phone to her softphone, or vice versa, the user will either have to call back the other person with whom she was speaking on her second device or ask the other person to call her back on the second device, often using a different phone number or user name/address for their second device. Manual switching between devices in the middle of a call is inconvenient for the users and disruptive to their conversations.